The present invention relates to a new type of metal matrix composite (MMC) and the process for manufacturing this new MMC. MMCs are well known structures, typically comprised of a ductile metal matrix, reinforced with ceramic fibers, whiskers, particulates, or dispersions. Most frequently, a prepared reinforcing material is mixed with molten matrix metal. Occasionally, the reinforcing structure is precipitated out of the molten phase of a melt consisting of compounds dissolved in the matrix metal.
These materials often share the best characteristics of both components of the matrix. They may combine the strength, hardness, corrosion resistance, and modulus of the reinforcement phase with the ductility, thermal and electrical conductivity, and machinability of the metal matrix phase. When aluminum is used as the matrix metal, the composite may be light, strong, and hard. This is important in many applications, specifically in machine parts, automotive and transportation parts, and electronic packaging.
Mixing a prefabricated reinforcing material with molten metal has the associated problems of inter-phase bonding, anisotropic characteristics, and non-uniform dispersion of the reinforcing structures in the matrix. Much effort has gone into solving these problems. The metal matrix does not always form a strong, cohesive bond to the reinforcing material. Methods have addressed improving both the mechanical and chemical bonding aspects, resulting in elaborately prepared starting material. For example, one technique first forms a composite of silicon carbide fibers within an alumina matrix, and then combines this composite with a metal matrix. This is done to obtain an adequate bond between the metal matrix and the silicon carbide fibers, using the alumina phase as an intermediary.
Other processes use layers or woven mats of reinforcing materials infused with molten metal. These structures have strongly anisotropic characteristics. Other fabrication techniques, such as hot or cold isostatic pressing, extrusion, and arc/drum spraying can also result in isotropic characteristics, depending on the type of reinforcing material. This results in a non-uniform material, which is undesirable in many applications.
Even without using processes that result in inherently anisotropic materials, uniform dispersion of the reinforcing phase within the matrix may result in a non-uniform material. For example, dispersed reinforcing particles may settle. One method that addresses this problem pounds the reinforcing phase into a powder of the metal, and then forms the finished part by sintering, which is a solid-phase process. Many other methods pre-form the reinforcing material into a near-finished shape and infuse it with molten matrix metal. However, obtaining a uniform infusion is difficult, as is obtaining a uniform bond between the matrix and the reinforcement phase, as discussed above.
The performance of the material is known to depend on its macroscopic mechanical properties, which must be uniform to achieve the uniform benefit of the composite. Moreover, a metal matrix that is not strongly bonded to the reinforcing phase does not gain the full value of the reinforcement. Finally, exotic, difficult, or complicated fabrication processes of either the composite or its precursor materials make the use of those composites economically unattractive, if not unfeasible. What is needed is a feasible and economically viable composite and method of fabrication.